present invention is concerned with a plant for the production of cellulose films, fibres, membranes or other moulded bodies according to the amine-oxide process, and an integrated film and fibre plant. Further, the present invention is concerned with a process for the production of cellulose films, fibres, membranes etc. according to the amine-oxide process.
From U.S. Pat. No. 2,179,181 it is known that tertiary amine-oxides are capable of dissolving cellulose and that from these solutions cellulose moulded bodies may be produced by precipitation in an aqueous medium. For the purposes of the present specification and claims, such a process is referred to as "amine-oxide process".
From U.S. Pat. No. 4,246,221, an amine-oxide process for the production of mouldable cellulose solutions using as starting material, among other substances, a mixture of cellulose in aqueous N-methylmorpholine-N-oxide (NMMO) is known. The known process is carried out discontinuously, presenting the following 4 steps for obtaining the mouldable solution:
1. processing a mixture of pulp containing approximately 6% by mass of water in an aqueous solution of NMMO in a mixing vessel by subjecting the mixture to reduced pressure and elevated temperature, a first solution being produced, PA1 2. intermediately storing the first solution in a tank, PA1 3. intermediately filtering said solution and PA1 4. processing said first solution in an extruder to obtain the mouldable solution. PA1 a mixing device for the production of a first suspension of pulp in an aqueous amine-oxide solution, which suspension has a dry substance density of not more than 10% by mass of dry pulp; PA1 a device for the concentration of the first pulp suspension produced in the mixing device, wherein the pulp suspension is mechanically spread like a layer on a heating surface, heated and transported across said heating surface while intensively mixing it and evaporating water, until a predetermined amount of water has evaporated and a concentrated pulp suspension is produced; PA1 a further device, preferably a thin-film treatment apparatus, wherein the produced concentrated pulp suspension is transformed into a mouldable solution of cellulose and PA1 an additional device connected to said further device or said thin-film treatment apparatus, wherein the mouldable cellulose solution is processed to cellulose films, fibres etc. PA1 a plant for the production of pulp; and PA1 a plant according to the invention for the production of cellulose films and fibres as described herein, said plant being provided downstream to the plant for the production of pulp. PA1 suspending pulp in an aqueous amine-oxide solution to produce a first suspension with a dry substance density of not more than 10% by mass of dry pulp; PA1 concentrating this first pulp suspension by mechanically spreading the pulp suspension like a layer on a heating surface, heating and transporting it across said heating surface while intensively mixing it and evaporating water, until a concentrated pulp suspension is produced; PA1 producing a mouldable cellulose solution by heating the concentrated pulp suspension and transforming it into a mouldable cellulose solution while evaporating water; and PA1 processing said mouldable cellulose solution in a known way to fibres, films or other moulded bodies;
According to this process, cellulose may be dissolved only discontinuously, which moreover is very time-consuming. The first step already takes 1 hour and 20 minutes. In this period of time, the pulp is exposed to the amine-oxide solution at a temperature of more than 100.degree. C., which leads to a significant degradation of the cellulose and the amine-oxide and to undesired by-products, deteriorating the properties of the produced fibres. Moreover, the big amount of amine-oxide processed per batch implies the risk of explosions, due to the thermal instability of the amine-oxide and degradation of the polymerisation degree of the cellulose may occur.
On the other hand, EP-A-0 356 419 published in 1990 describes a technique whereby it was possible to overcome the multi-step extruder technique, as for instance described in U.S. Pat. No. 4,246,221, and transform a suspension of cellulose in an aqueous solution of an amine-oxide into a mouldable solution continuously and even in one single step. According to this technique, first the suspension is spread like a layer on a heating surface while being intensively mixed and treated by transporting it, while intensively mixing it, across the heating surface, the layer heating up. Simultaneously, the layer is subjected during the treatment to a reduced pressure in such a way that water evaporates and those concentration conditions are adjusted which according to the phase diagram for the ternary substance mixture cellulose/amine-oxide/water (see e.g. WO94/28212) allow for the cellulose to dissolve.
The thin-film treatment process described above conveniently is carried out in an apparatus as illustrated in EP-A-0 356 419. This apparatus is called a thin-film treatment apparatus. An embodiment of a thin-film treatment apparatus is for instance a so-called FILMTRUDER.RTM., as manufactured by the company Buss AG (Switzerland). A thin-film treatment apparatus is also described in DE-OS 2 011 493.
WO 94/06530 published in 1994 uses the thin-film technique known from EP-A-0 356 419 to obtain a mouldable solution from a mixture of cellulose in an aqueous solution of a tertiary amine-oxide. The process is carried out in a FILMTRUDER.RTM., analogously to an embodiment described in EP-A-0 356 419. The process of WO 94/06530 has as its object to save energy and proposes for this purpose to rotate the rotor more slowly.